DESCRIPTION: Propagation of HIV in the human host requires cell entry, reverse transcription of viral RNA, integration into the human genome, transcription of the integrated provirus, and assembly/release of new virus particles. Currently there are antiretrovirals (ARTs) against each of these viral steps, except for provirus transcription. Although combinations of existing ARTs control HIV replication in most patients, drug toxicity and drug resistance are remaining concerns, arguing for the discovery of additional ARTs with novel mechanisms of action. In particular, an inhibitor of HIV transcription might both increase potency of treatment and suppress drug-resistant strains, improving the lives of patients. Our long-term goal is to improve treatment in HIV patients by targeting cellular factors essential in the virus life cycle. Cellular cyclin-dependent kinase 9 (CDK9) is required for transcription of both cellular genes and the HIV provirus. Approaches targeting CDK9 in vitro with catalytic inhibitors, RNAi, and direct inhibition using a dominant negative form, have all suggested that inhibition of HIV transcription without toxicity might be possible. Because HIV therapy is life-long, it is critical to determine safety and antiviral effectiveness of prolonged CDK9 inhibition in chronic HIV infection. We, and others, have previously shown that Indirubin 3'-monoxime (IM), a derivative of an ingredient in Chinese traditional medicine, inhibits CDK9 and HIV expression in primary lymphocytes and macrophages without cytotoxicity. In Preliminary Studies we show that IM suppresses plasma HIV RNA in NSG mice transplanted with human lymphocytes in the absence of toxicity, providing the first evidence for HIV inhibition by a CDK9 catalytic inhibitor in vivo. IM alone suppressed HIV RNA by > 2 log10 units, a magnitude of HIV reduction similar to that achieved with the NRTI EFdA in humanized mice. We also show that IM and ARTs from the NRTI, protease and integrase inhibitor classes have favorable anti-HIV interactions in vitro, suggesting IM could be used in combination with current ARTs. The goal of this application is to assess the anti-HIV potential of CDK9 inhibitors by evaluating antiviral mechanism, drug resistance and long- term toxicity in humanized mice chronically infected with HIV. Our hypothesis is that chronic treatment with CDK9 catalytic inhibitors can safely inhibit HIV transcription in vivo. We will test this hypothesis using IM or, alternatively, two novel CDK9 inhibitors. There are two Specific Aims. Specific Aim 1: To evaluate toxicity, pharmacokinetics, and antiviral activity of CDK9 inhibition in NSG mice transplanted with human CD34+ cells (HSC-NSG mice). Specific Aim 2: To evaluate mechanism of antiviral activity and long-term control of HIV in HSC-NSG mice treated with a CDK9 inhibitor. This proposal will assess the potential of blocking HIV transcription by prolonged treatment with a CDK9 inhibitor in chronically infected mice, resembling life-long therapy of HIV in patients. Successful testing of our hypothesis could improve HIV therapy by effectively targeting virus transcription.